Ethylene is commercially prepared by thermally cracking ethane at temperatures of 600.degree. to 1000.degree. C. The reaction, being endothermic, is costly due to high energy input and because the reaction vessels require special materials. At such temperatures, the reaction time is very short making the efficient recovery of heat from the process stream difficult. And, production of by-products adds to the difficulty and expense of ethylene recovery.
A process for the oxidative dehydrogenation of olefins is set forth in U.S. Pat. No. 3,856,881. This patent employs a catalyst containing vanadium, oxygen and at least one other metallic element. The patent states that a face-centered cubic form of crystalline structure is preferred and exemplifies the disclosure with vanadites of spinel structure. Phosphorus, is an optional element and, when present, the patent states that it should be in an amount of from about 0.002 to 0.35 atom of phosphorus per atom of vanadium, and preferably between about 0.005 and 0.20 atoms of phosphorus per atom of vanadium.
The patent states that the vanadium spinel compositions exhibit a certain type of X-ray diffraction pattern, one which does not have any sharp X-ray diffraction reflection peaks typical of a highly crystalline material having the same chemical structure. While this is true for vanadium spinel compositions, we have found such structures poorly suited for the oxydehydrogenation of ethane.
Ethane can be oxydehydrogenated to form ethylene with various oxyhalogenation catalyst systems at temperatures of 500.degree. to 600.degree. C. However, the presence of the halogen increases the recovery cost of the ethylene and requires special materials that withstand corrosion for construction of the reactors.
One process is known, however, for the oxydehydrogenation of ethane to ethylene at relatively low temperatures, which does not require special reactor materials and which provides relatively high levels of conversion, selectivity and productivity. This process involves the use of catalysts comprising mixed oxides of molybdenum and vanadium, together with a transition metal oxide; one optimum catalyst has the composition Mo.sub.0.61 V.sub.0.31 Nb.sub.0.08 O.sub.x. Preparation and use of this catalyst is discussed in U.K. Pat. No. 1,538,107 as well as a paper from Union Carbide, E. M. Thornsteinson, T. P. Wilson, F. G. Young and P. H. Kasai, J. of Catalysis, 52, 116-132 (1978). The authors reported a selectivity to ethylene of 83% at 25% conversion of ethane under atmospheric pressure and at 340.degree. C.
While the oxidation of paraffins is difficult, those having at least four carbon atoms are more reactive to catalytic conversion than is ethane. Butane, which is far less refractory than ethane, is converted to maleic anhydride in the presence of certain vanadium-phosphorus catalysts with various promoter elements. These catalysts are described in several U.S. patents of which we are aware, viz., U.S. Pat. Nos. 4,002,650 and 4,172,084, commonly owned by the Assignee of record herein, and U.S. Pat. No. 4,043,943. Notwithstanding the use of these catalysts for conversion of butane, we are unaware of their use as a catalyst for the oxydehydrogenation of ethane.